In Alzheimer’s, Nature and Nurture Play Dueling Roles

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December 21, 2003

Having a family member with Alzheimer’s disease may increase your risk of developing the mind-robbing ailment, since you may inherit certain genes that make you more susceptible to it. But a new Swedish study reveals environmental and lifestyle factors may play an equal or greater role in determining whether you eventually succumb to the memory loss and disorientation of Alzheimer’s.

Researchers from the Karolinska Institute, an esteemed medical research center in Stockholm, examined the relative contributions of genes and the environment among the very old. They enlisted 662 pairs of twins, both identical and fraternal, ranging in age from 52 to 98, and did follow-up tests for an average of five years.

“Because we know that identical twins share all their genes in common and fraternal twins have half their genes in common, we can use this information to quantify how important genes are,” said lead author Nancy Pedersen, Ph.D. “The added advantage of twins is that they are the same age, and we don’t have problems with waiting until other family members have passed the age at risk for the disease.”

During the five-year study, about 5.8 percent of the participants developed symptoms of Alzheimer’s. Of the identical twin pairs who shared the same genes, the other twin developed the disease about one third of the time. Among non-identical twins, less than one in ten did.

Statistical analysis revealed that genetic factors accounted for half or less to the onset of Alzheimer’s. Earlier studies of twins reported that genes contributed as much as 75 percent to the risk of the disease. Those investigations, however did not look at very old patients, an age group that is most susceptible to developing dementia.

Environmental factors, in this context, begin at birth and refer to a broad category of activities. These may include everything from possible exposure to various viruses and bacteria, what you eat and your typical diet, deficiencies in particular vitamins or nutrients, a history of head trauma, or harm from environmental toxins and pollutants.

In addition, earlier research found that staying physically active and other life events also affect risk. The more years of formal schooling you have, and the more challenging the hobbies and intellectual activities you engage in late in life, for example, the lower your risk of developing Alzheimer’s, studies have shown.

Researchers are still trying to unravel the roles of these and other and environmental factors in the onset of Alzheimer’s. It is likely that variations in many different genes likely interact with the environment to cause Alzheimer’s.

Genes vs. the Environment

Still, it is impossible to determine the exact extent to which any particular case of Alzheimer’s is due to nature (genes), nurture (environmental factors), or both. Scientists have identified several genes that contribute to the onset of Alzheimer’s. Most of these mutated genes cause a rare form of Alzheimer’s that runs in families and often strikes younger adults under the age of 65–sometimes as early as the 30s.

The far more common form of Alzheimer’s strikes seniors in their late 60s, 70s, and beyond. It has been linked to a gene called ApoE4. Gene experts can test for this gene and tell you if you have an increased risk for developing the mind-robbing ailment late in life. But just because you carry the gene doesn’t mean you will develop dementia for sure. About 60 percent of those who inherit the ApoE4 gene are likely to develop Alzheimer’s by age 70. Some with the gene may carry on for decades with their memories intact.

Additional genes that contribute to the disease will likely be identified. The National Institute on Aging is now recruiting family members with multiple Alzheimer’s victims for a study of the role of genes in the disease.

The study appeared in the December 15, 2003 online issue of the Annals of Neurology, a medical journal for doctors. It will be published in the February 2004 issue of the print edition.